1. Field of the Invention
The present invention relates to a transmission for a straddle type vehicle and, in particular, relates to an electronically controlled transmission that uses an actuator to engage and disengage a clutch and to perform a shift operation of a gear box, and a control method for the transmission.
2. Description of Related Art
Manual transmissions typically have a clutch lever for engaging and disengaging a clutch, and a shift pedal for performing gear change. When a shift change is performed, the rider must operate both a clutch pedal and the shift pedal. As a result, there is demand for a transmission that allows a shift change to be performed more simply, and that is easier for the rider to use.
In light of this demand, a manual transmission has been proposed that allows both engagement and disengagement of a clutch and a gear change to be performed by foot operation of a shift pedal that is connected to a shift shaft (transmission shaft). Thus, the rider can perform a shift change by just operating the shift pedal. Further, the transmission has a relatively simple structure and low cost.
In order to improve ease of use for the rider still further, an electronically controlled transmission has also been proposed in which the shift shaft is driven by a drive motor. For example, JP-A-11-082709 and U.S. Pat. No. 6,085,607 propose an electronically controlled transmission in which a shift shaft is driven by a single drive motor. In this electronically controlled transmission, a shift change is performed just by a simple switch operation. Operations that require great proficiency, like partial clutch engagement, are not necessary. With this electronically controlled transmission, the shift shaft is driven by the single drive motor. As a result, the electronically controlled transmission is simple and has a low cost.
However, the inventors of the present invention have found that if a vehicle mounted with the electronically controlled transmission described in JP-A-11-082709 and U.S. Pat. No. 6,085,607 is run continuously, there is variation in the magnitude of the shift shock that occurs during shifting. More specifically, as the vehicle is continuously run, the shift shock becomes relatively large as compared to immediately after start up.
With a manual transmission having a foot-operated shift pedal, a rider who is highly proficient in the shift operation can sense variation in the magnitude of the shift shock during continuous running, and can rapidly adjust the shift pedal operation in accordance with this variation. As a result, the magnitude of the shift shock does not vary that significantly.
It is possible to reduce variation in the magnitude of the shift shock during continuous running even with an electronically controlled transmission. More particularly, the shift operation can be minutely adjusted in accordance with a plurality of sensors provided in the transmission. However, as described above, the key features of an electronically controlled transmission that has a shift shaft driven by a single drive motor are ease of use and simplicity. If a plurality of sensors is disposed in the transmission, the structure becomes complicated and simplicity is lost.